1. Field of the Invention
This invention relates to a vertical carbonizing furnace for use in the production of carbon fibers.
2. Description of the Prior Art
In the past, it has been known to pre-carbonize, by oxidation, fibers composed of organic polymers such as polyacrylonitrile, cellulose, pitch, etc. and subsequently carbonize thus pre-carbonized fibers in an inert gaseous atmosphere in a carbonizing furnace to thereby, through continuous operation of the processes, obtain carbon fibers remarkably useful as reinforcing constituent, heat- or cold-resisting material, etc.
Carbon fibers are produced through the carbonizing process such that the pre-carbonized fibers are carbonized by heat in an inert gaseous atmosphere at temperatures in the range of 700.degree. to 1,800.degree. C., or more.
A conventional vertical carbonizing furnace is, as shown in U.S. Pat. No. 4,020,273, a vertically extending hollow cylindrical furnace and the pre-carbonized fiber subjected to oxidation (hereinafter referred to as "precursor") is passed through the inside of the furnace while a suitable tension is applied to the precursor. Normally, the carbonizing furnace is vertically provided with a temperature gradient by means of a plurality of heaters and it improves the physical properties of the carbon fibers. A conventional carbonizing apparatus is constituted of a plurality of these vertically extending hollow cylindrical furnaces and one precursor is passed through the central hollow portion of each furnace. According to such a conventional carbonizing apparatus, however, in case of the simultaneous processing of a plurality of precursors, it is necessary to have the same number of cylindrical hollow furnaces as that of the precursors, thus making the apparatus very large. In addition, there is a deficiency that heaters are required to be provided on each furnace thereby causing consumption of a large amount of energy. Therefore, the above-mentioned carbonizing apparatus is not suitable for use in the mass-production of carbon fibers.
Further, with respect to the vertically extending hollow cylindrical furnace, since a vertical conduit in the central portion of the furnace must be filled with an inert gas, the upper and lower ends of the conduit must be sealed. Regarding an effective sealing method to seal the furnace without hindering the precursor from continuously entering and leaving the furnace, as shown in UK Patent Publication No. GB 2059406A, a liquid sealing method requiring immersing one end of a tubular element connecting with the conduit into the liquid and a gas sealing method requiring the blowing of gas for sealing, into the end of the conduit, have been known. With regard to the above mentioned sealing methods, however, it is problematical that in the case of liquid sealing method, if the evaporated liquid enters the furnace, it will deteriorate the fiber in the the furnace or the furnace core. In case when the inert gas is discharged together with the evaporated liquid into the environmental air, it sustains a loss of the inert gas. Moreover, there is also a problem that a great amount of inert gas for sealing is required for the gas sealing method.